1. Field of the Invention
This invention relates to a frequency-to-voltage converter for converting the frequency (F) of the input signal from a rotational system to a voltage (V), which converter is applicable to automobile meters such as a speedometer and a tachometer.
2. Description of the Related Art
FIG. 10 of the accompanying drawings shows a meter driving circuit employing a conventional frequency-to-voltage converter.
As shown in FIG. 10, a frequency-to-voltage converter 10 comprises a pulse generator 12 and an integrator 14. The pulse generator 12 includes a wave shaping circuit 16 and a monostable multivibrator 18, generating a pulse signal for each input signal. This pulse signal has a constant width regardless of the frequency of the input signal. The integrator 14 includes a capacitor C and a resistor R, integrating the pulse signal to generate a signal voltage. Thus the frequency (F) is converted into the voltage (V).
In FIG. 10, reference numeral 20 stands for a sine wave generator to generate a sine variable having the value according to the voltage of the signal generated by the integrator 14. A driver 22 outputs the modulated SIN output defined by sin .theta. to a pointer driving coil 24.
A cosine wave generator 26 generates a cosine variable having the value according to the signal voltage of the integrator 14. A driver 28 outputs the modulated COS output defined by cos .theta. to a pointer driving coil 30.
The pointer driving coils 24, 30 are arranged to cross each other, moving the pointer according to the angle .theta. of the sine wave and cosine wave outputs, respectively.
However, such frequency-to-voltage converter suffers from a problem that the output voltage of the integrator 14 has a ripple component. When an input signal to the frequency-to-voltage converter has a low frequency, the ripple component in the output becomes large, moving the meter pointer by the low frequency of the ripple component. To diminish the ripple component, it is enough to enlarge a time constant which is a product of the capacitor C and the resistor R of the integrator 14. However, such measure will reduce a response speed of the meter. Otherwise, the ripple component would become large, moving the meter pointer excessively.